Electron accelerator



April 13, 1954 R. w|DERE ELEcTRoN ACCELERATOR 2 Sheets-Sheet l FiledApril l2. 1949 KSK April 13, 1954 R wlDERE 2,675,470

' ELECTRON ACCELERATOR Filed April l2. 1949 2 Sheets-Sheet 2 PatentedApr. 13, 1954 ELECTRON ACCELERATOR Rolf Widere, Zurich, Switzerland,assignor to Aktiengesellschaft Brown, Boveri & Cie, Baden, Switzerland,a joint-stock company Application April 12, 1949, Serial No. 87,021

Claims priority, application Switzerland July 28, 1948 1o claims. (c1.25o- 27) The present invention relates to devices for acceleratingelectrons to high velocity and high energy levels which are useful ingeneral nuclear research activities, therapy and radiography.

One type described in U. S. Patent No. 2,103,303 to Steenbeek that hasproven quite satisfactory in the past few years operates on theprinciple oi magnetic induction and is popularly referred to as abetatron or ray transformer. It is cornprised generally of an annularevacuated tube into which electrons are periodically injected from anelectron gun type of cathode in timed relation to a magnetic system thatproduces a magnetic field varying comparatively slowly with time (i. e.of the order of 50 cycles/sec.) and having a space distribution suchthat the electrons are accelerated by the eld along a circular orbit.The magnetic iield divides into two components. One of these known asthe inducing iield is established in a central core arranged coaxiallywith the tube and effects the electron acceleration; the other known asthe control field is established in annular pole pieces arranged aboveand below the tube and exerts a centripetal force upon the electrons tooffset the centrifugal forces resulting from the motion around thecircular path.

Due to the small mass of the electrons, the latter have been found toattain about 98% of their maximum possible speed, i. e. the speed oflight by the time their energy has reached the two million electron voltlevel. Additional energy applied to the electrons thereafter is nottherefore mani- .tested so much in increased velocity but rather mostlyas an increase in mass of the electron.

The mass cf the electron does not begin to increase appreciably untilthe velocity curve begins to level ofi' rather atly at about 95 of themaximum. Thereafter, the mass rises steadily with an increase in appliedenergy so that when the electron has reached an energy level of about100 million volts, its mass will have been increased to about 197 timesover that which it had at rest.

One disadvantage of the betatron is that the size oi the magneticstructure which carries the magnetic `held varies with the energy levelde sired to be reached. Hence the higher the nal energy level the largerand more expensive becornes the magnetic structure. A

To overcome disadvantage, it has been proposed te inodiiy the betatronby incorporating a high ireducncy gap in the annular tube and using thecit-ect or" the oscillating electrostatic field produced across the gapin combination with the time varied control held to impart additionalenergy to the electrons after the latter have been initially acceleratedto an energy level of about two million electron volts by the magneticinduction principle 0f the betatron at which time the inducing held coresaturates and betatron action ceases. An electron accelerator operatingon these combined principles is now commonly known as a synchrotron inthe synchrotron the frequency of the gap is of lcourse determined by theradius of the electron orbit since its period must correspond to thetime required by the electron stream to make one revolution around itsorbit. During the initial phase of the acceleration, the electrons arespread around the orbit but when the high frequency held is switched in,the electrons become hunched together and all reach the gap when the farside thereof is charged positively and are thereby drawn across the gapwith an attendant gain in energy. Thereafter, the electrons gainincrementally in energy each time they complete a revolution aroundtheir orbit and recross the high frequency gap and llnally are divertedfrom the orbit to be used for the particular purpose desired.

When the high frequency eld is operating, the system is practicallyself-stabilizing and the electrons will remain locked in phase with theiield so far as their time of arrival at the gap is concerned. Thus, forexample, should any electron tend to arrive late at the gap andconsequently receive too little energy, the magnetic control iield willurge the electron radially inward from the normal orbit and it will thenfollow a path of less radius, for the next revolution. However, becauseof the smaller path radius it will arrive at the high frequency gapsooner than the other electrons and 4consecpiently receives more thanits share oi energy from the eld with the result that the next timearound the orbit the path radius will tend to increase thus returningthe electron closer to its normal orbital path.

A similar stabilizing action takes place in the event that an electronriding the normal orbit arrives too early at the gap and receives moreenergy than it should. Its path radius then tends to increase during thenext revolution with the result that it tends to arrive late at the gap;lreceives less than its share of the increase in energy and the magneticcontrol eld pushes it back in the direction of the normal orbit.

The synchrotron which has been described above has, however, one seriousoperating disadvantage Which stems from the fact that the central coreof the magnetic structure' approaches saturation as the magnetic fluxthrough it increases with time to impart accelerating forces to theelectrons. Because of this saturation characteristic, the inductance ofthe energizing winding has an unavoidable variable characteristic andthus the variable wattless power taken by it cannot be entirelycompensated out through use of a bank of condensers since the lattercannot, in any practical manner, be likewise given a correspondinglyvariable capacity characteristic. One solution to the problem has beento load the relatively low frequency alternating current source feedingthe energizing winding for the synchrotrons magnetic structure withcomparatively high amplitude currents that originate from harmonics ofmuch higher frequency. However, these harmonies cause large losses andlarge drops in voltage and likewise a considerable increase in theWattless power requirements of the system.

The object of this invention is to provide an improved construction forthe magnetic structure of the synchrotron which makes it possible toovercome the disadvantages inherent in the conventional constructionwherein both the inducing and control magnetic elds are produced fromthe same winding. According to the present inventive concept, therelatively low frequency power source is used only for the purpose ofproviding the time varied magnetic control field component for keepingthe electrons on their orbit, and the magnetic inducing field essentialto betatron operation is supplied by a current of a relatively higherfrequency.

Several practical constructions embodying the inventive concept areillustrated in the drawings. Fig. l is a view in vertical centralsection of one embodiment; Fig. 2 is a plot of magnetic ilux curvesrelated to the Fig. 1 construction; Fig. 3 is likewise a view invertical central section of a slightly modiiied construction; Fig. 4 isa plot of magnetic ilux curves related to the Fig. 3 construction; andFigs. 5 and 6 are views in half central vertical section illustratingmodifications of the Fig. 3 construction.

With reference now to Fig. l, the synchrotron is seen to include acylindrical array of substantially C-shaped laminated plates I ofmagnetic material, a toroidly shaped, evacuated glass tube 2 locatedbetween the confronting annular pole faces 3-3 formed by the inner legs4, 4 of the C-shaped plates, and an energizing winding split into twoannular coil sections 5, 5 located in the space between the inner legs4, t and the outer legs 6 which form the yoke for completing the closedpath of the magnetic flux through the plates.

'I'he coil sections 5--5 which are arranged in series are connected to asource 1 of alternating current of relatively low frequency or. theorder of 50 cycles/sec., and a wattless power condenser 8 is connectedacross the coils 5-5'.

Electrons are emitted periodically in timed relation with thetime-Varied magnetic eld set up in the magnetic structure from anelectron gun 9 located to one side of the orbit 7c in tube 2 along whichthe electrons are to be accelerated, and the tube 2 includes anoscillator driven resonant cavity section which provides a highfrequency gap across which the electrons are pulled each revolution forincreasing their energy after the initial acceleration by the magneticinduction principle (betatron operation) `has terminated. However, sinceneither the specific construction of the cavity nor the manner forswitching in the same are of concern here insofar as the presentinvention is concerned they have accordingly not been included in theinterest of simplifying the drawings. One practical form, however, isshown and described in an article entitled Atomic Artillery that appearsin the June 1947 issue or the General Electric Review.

In the normal construction for a betatron type of accelerator, themagnetic structure also includes a central core extending axially alongthe axis A-A in which the inducing field is produced by the magnetizingcoils. However, in Fig. l it will be observed that due to the C-shapedconnguration of the laminated plate-s I, the magnetic structure whenassembled from the plates establishes an axial, cylindrically shapedopening I0. Consequently, the coils 5--5 are used only for the purposeof establishing the control field across the confronting annular polefaces 3-3 which maintains the electrons on the orbit lc.

In accordance with the present invention, the inducing field at higherfrequency is set up in a separate magnetic structure II which asillustrated has the form of a hollow rectangle made up from laminatedplates. One leg Ila of the core passes axially through the opening Illand has a coil I2 wound thereon which is connected in series with theenergizing coils v5 5. The winding I2 has the same number of turns ascoils 5--5' combined and its winding direction is opposite to that ofthe coils 5 5 thus establishing a magnetic eld which not only opposes orbucks the eld produced by coils 5--5 but is of the same magnitude so asto prevent the latter from setting up any inducing field component inthe separate core leg IIa. Thus the control field and the inducing eldare maintained entirely independent of one another. Consequently thecontrol iield which threads the tube 2 between poles 4--4' closesthrough the outer legs 6 as indicated by the flux path 4) depicted inFig. 1.

The inducing eld to be set up in the core leg IIa is produced by a coilI3 which surrounds leg I Ib of the separate core II. Coil IS connectedvia switch I4 to a source I5 of alternating current of the same phaseand frequency as the source 'l and a condenser I5 is connected acrossthe source I5. Although for simplicity illustration, switch I4 isillustrated as being of the blade type, the switch in practice would beof the thyratron or similar type as shown in 3 and so controlled that itcloses at the instant when the magnetic control flux produced in thecontrol poles 4 4' passes through the zero point in its cycle and atwhich instant the electron stream is injected into the tube. This zeropoint on the control iiux curve occurs when the voltages at the sources6 and I5 reach their maximum value. Condenser I6 which in the meantimebeen charged from source I5 while switch I4 was open, now dischargesthrough winding I3 with a 'frequency determined by the capacity ofcondenser I6 and inductance of coil I3 and produces in the core II, andparticularly the core leg IIa, the magnetic iiux a which serves yas theinducing field component for effecting initial acceleration of theelectron stream.

As is evident from the plot of curves in 2, the wavelength of theinducing field, i. e. magnetic flux q curve set up in the core II isabout one fourth that of the control field, i. e.. magnetic flux curve1, is constituted by a succession of pulses; having a 'repetition rateequal -to the frequency -of the; control field and attainsra much lowerA.maximum amplitude. However, in the neighborhood of the zero point onthe flux curves, both fluxes gb and v151y have substantially the sameslope and vary in like manner with time, so that in this neighborhood,the necessary 1:2 relation between the control and inducing rfieldcomponents essential to operation on the beta-tron prin-- ciple, andwhich is explained in the aforementioned Steenbeek patent, can beestablished without diiilculty through proper selection of the peakamplitudeof the voltage at source |5-andA the capacity of condenser t6.

Fig. 3 shows another practical construction for asynchrotron embodyingthe principles of the invention. In this modification which is adaptedto accelerate electron streams in opposite directions aroundorbit k onsuccessive halves of the alternating current wave applied to theenergizing coils, the magnetic structure of the synchrotron differsfromvthat shown in Fig. l in that the central leg l8- in which. the inducingfield is produced is structurally united with the control `poles IS--IS'by means of inner yokes 20--26 in addition to the outer yokes 2i. Thecentral leg I8 is surrounded by a coil 22 and the latter contains a fewless turns than the magnetizing coils 23-23 to-assure that none of theflux produced by the latter coils closes through the central leg I8.That is, all of the flux which passes through the control poles lil-IS.is returned via the outer yokes 2 I. only.

Coils 22 and 23--23 are arranged in series and connected to a source ofalternating current 24 of relatively low frequency, .and a Wattlesspower condenser 25 is connectedacross the coils in the same manner andfor the samey purposeas condenser 8 in the-Fig. l construction.

The circuit for producing the higher frequency current for establishingthe inducing field in the central leg I8 is comprised of anYautotransformer 25 connected across the alternating current source 24, acondenser 2.1 connected for energization periodically from theautotransformer, and a pair of thyratron tubes 28, 29 connected inparallel and arranged in front-to-back relation for periodicallydischarging the condenser 2l through coil 22'. Condenser 21 thuscorresponds in function to condenser I6 in Fig. l.

For switching in the thyratrons alternately at the proper instant ineach half wave of the magnetic control flux produced in the controlpoles lr--IB by coils 23-23, a saturable core peaking transformer 30 isutilized. The latter includes a primary windiru.;` 3l on core 32connected in series with coils 22, 23-23 and two secondary windings 33,343, Wound in relatively opposite directions. One side of winding 33 isconnected through a direct current biasing battery 35 to 'the cathode23a of thyratron tube 28 and the other side therecf to the control grid28h of such tube; the grid 29h and cathode 29a. of thyratron 29 areconnected in a similar manner through biasing battory 3S to the terminalends of the transformer secondary 34.

Operation ofthe arrangementv shown inFig. 3 is the samein principle asthat illustrated in Fig. l except that the former is adapted to operateon both halves of the magnetizing vflux wave as previously mentioned.Referring now to the curves plotted` in Fig. 4, the flux wave throughthe control poles: lS-IS and which is produced by coilsv 2:3.-23"isdesignated by curve Q53. As this wave passes through thezeroipointat.time aat; whichtime a stream of.y electrons is injected fromvr cathode31, transformer 30 will unsaturate momentarily producing peaked voltagesin the secondaries, 33, 34 of relatively opposite polarity. Thesevoltages are of course impressed on the grids of the thyratrons 28--29and since the latterl are arranged front-to-back, only one of them suchas tube 28, will then iire and discharge condenser 21 through the coil22 thus producing the inducing ux qu. in the central leg I8 at thehigher frequency and lower amplitude as indicated in Fig. 4.

In a similar manner, as the control field flux wave es next passesthrough zero, i. e. at the time t1, and at which timeanother stream ofelectrons is injected in the opposite direction from cathode 3lk .foracceleration in the opposite direction around tube 3-8, transformer 3i!will again produce peaked voltages in the secondaries 33, 34 causing theother tube 29 to fire and discharge condenser 21, which in the meantimehas recharged, in the opposite direction through coil 22.

It will be observed that a resistor 39 is connected in the circuitbetween the thyratrons 28, 29 and one side of the coil 22. The effect ofthis resistor is to. flatten out the slope of the flux curve c4. Bymaking resistor Sii suiiiciently large, it becomes possible to empty thecondenser 2l completely of its charge each time that discharge takes`place. This is especially desirable when only half wave operation of theaccelerator is desired, in which case, only one thyratron switchingdevice would then be required.

If itis desired to use the same number of turns l on coil 22as'are incoils 23, 23', either an inductance 4Q must be connected in parallelWith the winding 22 as shown in Fig. 5, or the central leg i8 must beinterrupted by an air gap 4I as shown in Fig. 6. Otherwise, the magneticstructurey coils and control circuits for Figs. 5 and 6 are the same asin Fig. 3.

In conclusion, I Wish it to be understood that while the constructionsshown in the drawings are preferred, various changes may be made in thedetails particularly as to the switching circuits, Without departingfrom the spirit and scope of the invention as defined by the appendedclaims.

I claim:

1. In a device foi` accelerating electrons to high energy levelsy and inwhich an initial acceleration to substantially the speed of light iseffected through magnetic induction, the combination comprising, anannular evacuated tube providing a closed orbital path for electrontravel, a magnetic structure associated with said tube and whichincludes a central core extending axially through said tube, a pair ofcontrol poles confronting each other on opposite sides of said tube atsaid orbital path and yoke means disposed radially outwardv of 'saidcontrol poles and connectedtherewith, a main winding surrounding saidcontrol poles and adapted to be energized by a first. alternatingcurrent of relative r' low. frequency lto produce a time varied magneticcontrol eld in said control poles, an auxiliary winding ysurroundingsaid central core and connected. in Vseries with said main winding forproducing a time varied magnetic ii'eld bucking and nullifying themagnetic iield which otherwise would be produced-y insaid central coreby said main winding whereby said control eld closes through said yokeVmeans rather than through said central core, `and means includingwinding means associated with said` central core arranged forenergization from a source of a second alternating current having awavelength substantially shorter than that of said first alternatingcurrent, said second alternating current being applied to said windingmeans in pulse form having a repetition rate equal to the frequency ofsaid first alternating current and which produces a time varied magneticinducing field in said central core in phase with said magnetic controlfield as the latter passes through the zero point in its wave.

2. An electron accelerator as defined in claim 1 wherein said secondsource of alternating current is constituted by a charged condenser.

3. An electron accelerator as defined in claim l wherein said secondsource of alternating current is constituted by a charged condenser andfurther including a resistor connected in circuit between said condenserand the winding associated therewith.

4. An electron accelerator as defined in claim l wherein said secondsource of alternating current is constituted by the periodic dischargefrom a condenser and said condenser is charged from an alternatingvoltage of substantially the same phase as the voltage wave of thealternating current source that produces said magnetic control field.

5. An electron accelerator as defined in claim 4 wherein the saidperiodic discharge of said ccndenser is effected through agrid-controlled electric discharge valve of the thyratron type and inwhich the grid voltage varies with and reaches the ignition point ofsaid valve when said time varied contro1 field passes through zero.

6. In a device for accelerating electrons to high energy levels and inwhich an initial acceleration to substantially the speed of light iseffected through magnetic induction, the combination comprising, anannular evacuated tube providing a closed orbital path for electrontravel, a magnetic structure associated with said tube comprising afirst closed body of magnetizable material providing a pair of controlpoles confronting each other on opposite sides of said tube at saidorbital path, yoke means disposed radially outward of said control polesand connected therewith, a central core extending axially through saidtube and which constitutes one leg of a second closed body ofmagnetizable material structurally independent of said first body, amain winding surrounding said control poles adapted for energization bya first alternating current of relatively low frequency to produce atime varied magnetic control field in said conf trol poles, an auxiliarywinding surrounding said central core and connected in series with saidmain winding for producing a time varied magnetic field bucking andnullifying the magnetic field which otherwise would be produced in saidcentral core by said main winding whereby said control field closesthrough said yoke means rather than through said central core, and athird winding on said second body of magnetizable material arranged forperiodic energization by a second alternating current having awavelength substantially shorter than that of said first alternatingcurrent, said second alternating current being applied to said windingmeans in pulse form having a repetition rate equal to the frequency ofsaid first alternating current and which produces a time varied magneticinducing field in said central core in phase with said magnetic controliield as the latter passes through the zero point in its wave.

"7. In a device for accelerating electrons to high energy levels and inwhich an initial acceleration to substantially the speed of light iseffected through magnetic induction, the combination comprising, anannular evacuated tube providing a closed orbital path for electrontravel, a unitary magnetic structure associated with said tubecomprising a central core extending axially through said tube andprovided with an air gap and a pair of confronting control polesdisposed on opposite sides of said tube at said orbital path, yoke meansdisposed radially outward of said control poles, a main energizingwinding surrounding said control poles and adapted to be energized froma first relatively low frequency alternating current to produce a timevaried magnetic control field in said control poles, an auxiliarywinding surrounding said central core, said auxiliary Winding having thesame number of turns as said main winding and connected in seriestherewith for producing a field in said core bucking and nullifying thefield which otherwise would be produced in said core by said mainwinding whereby said control field closes through said yoke means ratherthan through said central core, a second alternating current having awavelength substantially shorter than that of said first alternatingcurrent and switching means actuated in timed relation with said firstalternating current for applying said second alternating current to saidauxiliary winding in pulse form having a repetition rate equal to thefrequency of said first alternating current and which produces a timevaried magnetic inducing field in said central core in phase with saidmagnetic control field as the latter passes through the zero point inits wave.

8. In a device for accelerating electrons to high energy levels and inwhich an initial acceleration to substantially the speed of light iseffected through magnetic induction, the combination comprising, anannular evacuated tube providing a closed orbital path for electrontravel, a unitary magnetic structure associated with said tubecomprising a gapless central core extending axially through said tube,a, pair of confronting control poles disposed on opposite sides of saidtube at said orbital path and yoke means disposed radially outward ofsaid control poles, a main energizing winding surrounding said controlpoles and adapted to be energized from a first relatively low frequencyalternating current source to produce a time varied magnetic controlfield in said control poles, an auxiliary winding surrounding saidcentral core, an inductance connected in parallel with said auxiliarywinding, said auxiliary winding having the saine number of turns as saidmain winding and connected in series therewith for producing a field insaid core bucking and nullifying the field which otherwise would beproduced in said core by said main winding whereby said control fieldcloses through said yoke means rather than through said central core, asecond source of alternating current having a wavelength substantiallyshorter than that of said first alternating current and switching meansactuated in timed relation with said first alternating current for`applying said second alternating current to said auxiliary winding inpulse form having a repetition rate equal to the frequency of said firstalternating current and which produces a. time varied magnetic inducingfield in said central core'in phase with said magnetic control field asthe latter passesthrough the zero point in its wave.

9. In a device for accelerating electrons to high energy levels and inwhich an initial acceleration to substantially the speed of light iseffected through magnetic induction, the combination comprising, anannular evacuated tube providing a closed orbital path for electrontravel, a unitary magnetic structure associated with said tubecomprising a gapless central core extending axially through said tube, apair of confronting control poles disposed on opposite sides of saidtube at said orbital path and yoke means disposed radially outward ofsaid control poles, a main energizing winding surrounding said controlpoles and adapted to be energized from a rst relatively low frequencyalternating current source to produce a time varied magnetic control eldin said control poles, an auxiliary winding surrounding said centralcore, said auxiliary winding having a few less turns than said mainwinding and connected in series therewith for producing a eld in saidcore bucking and nullifying the field which otherwise would be producedin said core by said main winding whereby said control field closesthrough said yoke means rather than through said central core, a secondsource of alternating current having a Wavelength substantially shorterthan that of said rst alternating current and switching means actuatedin timed relation with said rst alternating current for applying saidsecond alternating current to said auxiliary winding in pulse formhaving a repetition rate equal to the frequency of said firstalternating current and which produces a time varied magnetic inducingeld in said central core in phase with said magnetic control field asthe latter passes through the zero point in its wave.

10. In a device for accelerating electrons to high energy levels and inwhich an initial acceleration to substantially the speed of light isefrected through magnetic induction, the combination comprising, anannular evacuated tube providing a closed orbital path for electrontravel, a magnetic structure associated with said tube and whichincludes a central core extending axially through said tube, a pair ofcontrol poles confronting each other on opposite sides of said tube atsaid orbital path and yoke means disposed radially outward of saidcontrol poles, a main Winding surrounding said control poles and adaptedto be energized by a first alternating current source of relative lowfrequency to produce a time varied magnetic control field in saidcontrol poles, an auxiliary winding surrounding said central core andconnected in series with said main Winding for producing a time variedmagnetic field bucking and nullifying the magnetic field which otherwisewould be produced in said central core by said main winding whereby saidcontrol field closes through said yoke means rather than through saidcentral core, and means associated with said central core arranged forenergization of said core with a second alterhating current having awavelength substantially shorter than that of said first alternatingcurrent and switching means actuated in timed relation with said rstalternating current for applying said second alternating current to saidauxiliary winding in pulse form having a repetition rate equal to thefrequency of said first alternating current and which produces a timevaried magnetic inducing eld in said central core in phase with saidmagnetic control iield as the latter passes through the zero point inits wave.

References Cited in the file of this patent UNITED STATES PATENTS NumberName Date 2,331,788 Baldwin Oct. 12, 1943 2,394,072 Westendorp Feb. 5,1946 2,465,786 Blewett Mar. 29, 1949 2,480,169 Westendorp Aug. 30, 19492,533,859 Wideroe Dec. 12, 1950 2,535,710 Westendorp Dec. 26, 1950

